01/29/2015 01:15 pm ET Updated Mar 31, 2015

Breaking Out: Jennifer Aniston Takes the Cake!

It is always inspiring to witness a rare act of transformation, especially when it involves someone we've come to know and admire. This month, that startling moment of truth is on display in movie theaters across America.

When Jennifer Aniston first appears onscreen in Daniel Barnz' unsettling new film, Cake, we are instantly aware that her character will be nothing like the irresistibly fetching Rachel she embodied for a decade on Friends. Although Jennifer has taken on many winning movie roles since Friends left the airwaves, stepping into the skin of Cake's Claire Simmons -- a cynical, complicated forty-something who is addicted to prescription painkillers as a result on an auto accident that has left her emotionally and physically scarred -- was no small feat for the actress: To inhabit the role, she gained weight, quit exercising, wore no makeup and hid her famously luxuriant head of hair under a less than flattering short cut.

The physical self-makeover alone is stunning, but Jen raises (and then clears) the bar even more by actually making us root for a character who, in lesser hands, would simply be a turn-off.

"If you're in someone's living room every week for 10 years," She told The New York Times, "people are going to have a hard time saying, 'O.K., so we're going to see you do what now?" Even more impressive than Jennifer's performance is the courage it took to step up to such a challenging assignment -- the leap earning her critical acclaim and a whole new fan base.

Of course, Jennifer isn't the first actor to make such a dramatic leap, but each time it happens it's exciting to see.

When the late Robin Williams morphed from sitcom star to movie actor, it took him just one light-hearted role (as Popeye) before finding his groove in The World According to Garp, a breakthrough performance that would lead him to a succession of even more demanding assignments -- from Dead Poets Society to The Fisher King to Good Will Hunting. Likewise Sally Field, whose fiery turn as union champion Norma Rae (1979) earned her an Oscar, proving once and for all that she wasn't Gidget anymore. And just last year, Matthew McConaughey turned his long reign as the Prince of Rom-Com on its ear with his gripping portrayal of a shrewd and hot-headed AIDS victim in Dallas Buyers Club -- a role for which the Academy named him Best Actor.

I know a little bit about stepping outside the comfort zone, too. When I ended my TV series, That Girl, I was determined to move onto roles that didn't feel quite so natural to me. Comedy had always come easy to me (thank you, Dad!), so I began looking for a part that would challenge me to dig deeper. I found that role in Nobody's Child, a television film based on the true story of a woman who'd made a harrowing journey out of mental illness. Playing Marie Balter was a very powerful experience for me, not only because she was an intense character, but also because I was able to spend time with the real Marie and learn her from the inside out, and I was rewarded with the Best Dramatic Actress Emmy for my performance. The experience confirmed to me that we need to push ourselves beyond our own pre-conceived boundaries and see where that takes us.

Which is exactly what Jennifer Aniston does in Cake. I've always been a big Jen fan, and not just because (full disclosure) I played her mom on Friends! She is a truly gifted actress -- funny, smart and wholly devoted to her craft -- and I really hope you'll see for yourself how she hits the bullesye in her latest and most demanding role.

In the meantime, take a look at the slideshow we've prepared for you, which includes some other breakthrough performances that you may remember.

Transforming Roles